1. Technical Field of the Invention
The present invention relates to digital wireless communication systems. More particularly, the present invention relates to Wideband Code Division Multiple Access (WCDMA) communication systems. Even more particularly, the present invention relates to data transmissions between user equipment (UE) and a receiver in a WCDMA system.
2. Description of Related Art
The physical layer in WCDMA offers one or several transport channels to the higher layers. Data from the transport channels are individually coded, multiplexed together, and transmitted over the air to the receiving end. The transmission time interval (TTI) for a transport channel is the duration of data over which coding and interleaving is performed. For Frequency Division Duplex (FDD) mode, this also corresponds to the actual transmission time over the air of the transport blocks in a given TTI. Currently, the WCDMA FDD uplink supports TTIs of 10, 20, 40, or 80 ms, a parameter that is semi-statically configured per transport channel via relatively slow higher layer signaling.
In a packet data transmission scenario, data typically arrives to the physical layer in the form of transport blocks having a fixed size, e.g., 336 bits. This size cannot be easily or rapidly changed and is typically fixed to the same value for all users in a system. One transport block is the smallest non-zero quantity of data that can be transmitted on a transport channel in one TTI, which gives the lowest non-zero data rate that can be supported. For a given transport block size, the longer the TTI, the lower the smallest non-zero data rate is. At the same time, low delays are usually important for packet data systems. Hence, the shorter the TTI, the better, from a delay perspective.
In a wireless communication system, the higher the data rate, the larger the received power has to be to maintain a suitable received energy per information bit. Thus, there is a maximum terminal-to-base-station distance where a certain data rate can be reliably supported. At this distance, the terminal is transmitting at maximum power to overcome the propagation loss from the terminal to the base station and still maintain the minimum required received power for reliable communication at the data rate considered. Thus, the further from the base station the terminal is, the lower the maximum data rate possible. Typically, a cellular network is planned for a certain minimum (uplink) bitrate, e.g., 64 kbit/s, by placing the base stations at a suitable distance from each other.
To ensure error-free delivery of received packets to the application layer, (wireless) communication systems typically use hybrid Automatic Repeat Request (ARQ). In a system using hybrid ARQ, data are coded and transmitted to the receiver. The receiver tries to decode the received data and, if errors are found in the received data, the receiver requests a retransmission of the data unit from the transmitter. If no errors are found in the decoding process, the received data unit is considered to be correctly received and the receiver transmits an acknowledgement signal to the transmitter and passes the received data unit to higher layers. Thus, (near) error-free delivery of data units to higher layers can be provided.
The performance of the hybrid ARQ mechanism can be further enhanced by performing soft combining, i.e., the receiver is buffering the erroneously received data unit and combines the buffered soft information with the soft information received due to the retransmission(s).
A simple illustration of the operation of an ARQ protocol is shown in FIG. 1. For illustrative purposes, this figure uses multiple independent stop-and-wait protocols in a similar way as is done for high-speed downlink packet access (HSDPA).
The transmitter transmits one unit of data in the first frame. Along with the data, control information is transmitted, e.g., hybrid ARQ process number and a new data indicator. Upon reception, the receiver tries to decode the received signal and transmits an ACK or NAK to the transmitter. In FIG. 1, the decoding process failed and a NAK is transmitted in order to request a retransmission from the transmitter. The transmitter retransmits the data, this time with the new data indicator set to indicate that this is a retransmission and the received signal should be soft combined with the already buffered information in order to improve the probability for successful decoding. The idea behind the process number is to be able to utilize multiple parallel stop-and-wait protocols. Thus, while trying to decode the data transmitted in frame 1 and intended for hybrid ARQ process 1, frame 2-4 can be used for transmission to other hybrid ARQ processes, e.g., process 2-4.
In Third Generation Partnership Project (3GPP), there are currently discussions on how to enhance the performance for packet data services in the uplink. One of the major concerns is to reduce the delays. A significant delay reduction is possible if hybrid ARQ with soft combining is introduced and placed in the base station, in which case the base station rapidly can request retransmission of erroneously received data units from the terminal instead of relying on slower, higher layer retransmission protocols. A further delay reduction is possible if the minimum TTI is reduced from 10 ms to 2 ms.
Currently, neither hybrid ARQ (located in the base station), nor 2 ms TTI is supported by the uplink in WCDMA, but the introduction is currently being discussed in 3GPP. The introduction of hybrid ARQ with soft combining is conceptually straightforward. A semi-static TTI of 2 ms can in principle be based on existing structures.
As stated above, the minimum non-zero data rate is higher when the TTI is shorter. Hence, assuming an unchanged maximum terminal transmission power, the coverage for terminals using a new 2 ms TTI may be affected compared to terminals using the existing 10 ms TTI. With the example numbers above, a minimum transport block size of 336 bits, which is the smallest non-zero unit that can be transmitted in the uplink, corresponds to a minimum non-zero data rate of 33.6 kbit/s for 10 ms TTI and 168 kbit/s for a 2 ms TTI. Assuming a network planned for 64 kbit/s coverage, which is a typical value, data transmission at the cell border can be guaranteed for a TTI of 10 ms, but not for a TTI of 2 ms. This is unfortunate as it is not desirable to re-plan the network when a new feature is introduced in the specifications. In addition to the coverage issues, there may also be reasons from a radio resource management (RRM) point of view to be able to use a minimum non-zero data rate significantly lower than 168 kbit/s.